This invention relates to a system for using power supplies in parallel. More particularly, it relates to such a system which provides load current sharing among the power supplies used while providing good voltage regulation at the load. Even more particularly, it relates to such a system which does not require that the power supplies used be identical and which will not cause an overvoltage condition at the load if one of the power supplies fails.
Power supplies are used to provide the required operating power to a piece of electronic equipment, termed the "load". Power (P) is a measure of both the voltage (V) applied to the load and the current (I) delivered to the load (P=V.times.I). Quite often, it is desirable to connect the output terminals of more than one power supply in parallel in order to supply the desired power. One reason for doing this is to increase the overall reliability of the electronic system. For example, if only one power supply is used, and if that power supply fails, then the entire system is non-operational until the power supply is repaired or replaced. However, if a number of supplies are used in parallel, and if the supplies have sufficient current capacity, then one or more of the supplies can fail without affecting the operation of the electronic system.
The electronic equipment using the power is usually not a static load, meaning that the amount of current required by the load is continually changing as various circuits within the equipment switch on and off. This continual change in current can cause the voltage at the load to change and, if the change is large enough, affect the operation of the equipment. Thus, regulating the voltage at the load is an important consideration, no matter how many power supplies are used.
Current sharing is a second important consideration when power supplies are used in parallel. Depending on how they are connected, one power supply may furnish all the current. If this is not possible, because the total current required by the load exceeds the current capacity of a single power supply, then one or more of the parallel power supplies may be operating at its respective maximum current capacity, while the remaining power supplies may furnish little or no current. If this unbalanced condition occurs, the power supplies that are operating at maximum current capacity, and hence those being stressed at maximum performance levels, are subject to an earlier failure than those not operating at maximum current capacity. Thus, poor current sharing among parallel power supplies can obviate the reason for operating them in parallel, i.e., increased reliability.
FIG. 1-4 illustrate some of the prior art schemes for operating power supplies in parallel. Only two power supplies are shown in each of the figures. However, the same principles apply, regardless of the number used.
FIG. 1 shows two power supplies 10-11 connected in parallel using local sensing. The squares on the power supplies labeled "+", "-", "+S" and "-S" represent the positive, negative, positive sense, and negative sense terminals, respectively, of the power supplies. The positive terminals "+" of each power supply are connected to one side of the load 12 by the cables 13 and 16, while the negative terminals "-" are connected to the other side of the load 12 by the cables 14 and 15. Each cable 13-16 has an inherent resistance, represented by the resistors RC1-RC4, respectively. Since the sense terminals "+S" and "-S" are not used, the internal circuitry will sense the voltage at the output terminals "+" and "-" to regulate the voltage at those terminals. Such operation is known in the art as "local sensing". (The manner in which a power supply uses the sense terminals to regulate voltage is well understood in the art of power supplies and need not be explained here.)
In operation, each power supply 10-11 is set so as to provide the desired voltage across the load. However, it is physically impossible to set both power supplies to exactly the same voltage, so there will always be some difference. The cable resistances, RC1-RC4, can be made very nearly equal by making each cable 13-15 the same diameter and same length. The voltage at the load 12 can, of course, have only one value because it is a single point.
When the power supplies 10-11 are turned on, each will supply a current that causes a voltage drop across the appropriate cable resistance that is equal to the difference between the voltage across the output terminals of the supply and the voltage across the load. Since the output voltages of the supplies are nearly equal, and since the cables resistances are nearly equal, the currents from each supply will be nearly equal. That is, current sharing is very good. However, voltage regulation at the load 12 is poor. As the current required by the load changes, the voltage dropped across the cable resistances changes, thus changing the voltage at the load.
FIG. 2 is a block diagram showing two power supplies connected in parallel using remote sensing. This figure is similar to FIG. 1 except now the sense terminals "+S" and "-S" of the power supplies 10-11 are connected to the load 12 by the wires 17,19 and 18,20 respectively. Since the voltage at the load is directly sensed, load voltage regulation is very good. However, as will be explained, current sharing is poor. Since the two supplies can not be set to exactly the same voltage, one of them will be set to a higher voltage than the other. This power supply will furnish all of the current (or up to its maximum capacity) in order to make the voltage drop at the load equal to its voltage adjustment setting. The remaining power supply will not have to furnish any current (or not nearly as much) for the load voltage to reach its voltage adjustment setting, as the voltage setting may already be exceeded with current from the other supply.
FIG. 3 is another prior art configuration showing two power supplies connected in parallel in a "master-slave" configuration. The master power supply 10 has its sense terminals connected to the load 12 by the wires 17-18. The remaining power supplies, the slaves, represented by the single power supply 11, do not have their sense terminals connected and thus operate in local sense.
The master power supply 10 of FIG. 3 has its output voltage set to the desired value, and because of remote sensing, provides very good voltage regulation at the load. The slaves 11 are set at a slightly higher voltage. The slave power supplies 11 have their current limit controls set such that they can only supply a desired fraction of the total current. The master supply 10 then provides very good voltage regulation and the remaining current required in the manner described in conjunction with FIG. 2. The disadvantage of this configuration is that it depends upon the continued operation of the master power supply to provide very good voltage regulation. If the master power supply should fail, the whole system will fail and possibly cause an overvoltage, undervoltage, or shut off condition to occur.
FIG. 4 shows two power supplies connected in parllel using forced current sharing. The figure shows a portion of the internal circuitry, the resistors R1-R4 and the PWM (pulse width modulator) 21, of each power supply 10-11. This internal circuitry uses the voltage on the sense terminals to control the output voltage. This control is achieved by using the sense terminal voltage to control the PWM. If the sense voltage changes, the width of an output pulse from the PWM changed in order to force the voltage back to a desired level. The detailed operation of the internal circuitry of the power supplies 10 or 11 is well understood in the art and need not be explained here.
As shown in FIG. 4, the inputs to the PWM's 21 of each power supply 10-11 are tied together by a wire 22. Thus, the voltage input of each PWM 21, is the same and their response is similar. Very good current sharing results from this configuration since the only difference in current is caused by the small differences in the cable resistances RC1-RC4. The major disadvantage of the configuration shown in FIG. 4 is that it requires all the operating parallel power supplies to have the same voltage sensing circuits therewithin. This is not always possible. (e.g., not all power supplies provide a means for connecting a jumper 22 between the voltage sensing circuits.) In many cases, the power supplies are purchased components and it is desirable to purchase them from a number of vendors in order to insure an adequate supply. When this is done, it is usually not possible to mix power supplies from different manufacturers, since their internal voltage sensing circuits are not the same.
Other schemes are known in the art to operate power supplies in parallel in addition to those shown in FIGS. 1-4. As an example, one such scheme monitors the current being supplied to the load by each power supply and electronically alters the voltage being sensed by the supply, which alternation steers the output voltage in a direction that maintains the current at the desired level. The primary disadvantage of this approach is that the means used to alter the voltage, usually an operational amplifier, is subject to fail in a manner that can cause the power supply to increase its output voltage beyond the safe operating limit of the load. Unfortunately, this overvoltage condition can cause the load to fail also.
It is apparent from the preceding discussion that a need exists in the art for a system of operating power supplies in parallel which provides good current sharing, good voltage regulation at the load, does not require that power supplies with identical internal voltage sensing circuits be used, and will not cause an overvoltage on the load if it should fail.